OMAPDSS: Add support for DPI source selection

We can select the video source for DPI output as follows:

OMAP2/3: always LCD1
OMAP4: LCD2 or DIGIT
OMAP5: LCD1/LCD2/LCD3/DIGIT

This patch adds support to select the source, and makes dpi.c call the
function to set the source.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Anders <x0132446@ti.com>

drivers/video/omap2/dss/dss.h

@@ -277,6 +277,7 @@ void dss_overlay_kobj_uninit(struct omap_overlay *ovl);
 int dss_init_platform_driver(void) __init;
 void dss_uninit_platform_driver(void);
 
+int dss_dpi_select_source(enum omap_channel channel);
 void dss_select_hdmi_venc_clk_source(enum dss_hdmi_venc_clk_source_select);
 enum dss_hdmi_venc_clk_source_select dss_get_hdmi_venc_clk_source(void);
 const char *dss_get_generic_clk_source_name(enum omap_dss_clk_source clk_src);